1. Field of Invention
The present invention relates in general to nuclear well logging, the pertains in particular to improved methods and apparatus for analyzing radiation energy spectra to provide more accurate information of the constituents of a geological formation-borehole system.
2. The Prior Art
Heretofore, various techniques have been utilized to process gamma ray energy spectra for formation constituent analysis. In the case of inelastic scattering gamma ray energy spectra, it is known that analysis of the spectra to identify the contributions thereto due to carbon and oxygen provides useful information of the presence of oil in a formation. Additional information concerning the composition of the formation, such as its lithology for instance, is however frequently required before an unambiguous determination of the presence of oil can be made. A suitable lithology indicator for this purpose might comprise the ratio of inelastic scattering gamma ray contributions for calcium and silicon.
The derivation of the foregoing information concerning carbon, oxygen, calcium and silicon, and possibly other constituents of the formation, depends upon accurate constituent analysis of the formation gamma ray spectra. An important and basic technique for performing such analysis is disclosed in U.S. Pat. No. 3,521,064, issued on July 21, 1970 to Moran, et al. In accordance with the Moran et al teaching, a detected gamma ray energy spectrum for a formation of unknown composition is compared with a composite spectrum made up of weighted standard spectra of the constituents postulated to comprise the formation. The weight coefficients for the standard spectra which give the best fit of the composite spectrum to the unknown spectrum, as determined, for example, by the method of least squares, represent the relative proportions of the constituents in the formation. By appropriate selection of the standard spectra, the proportions of the constituents of interest, such as carbon, oxygen, calcium, silicon, etc., may be obtained, from which the desired information regarding oil content may be derived.
It has further been proposed in U.S. application Ser. No. 869,584, filed Jan. 16, 1978, for R. C. Hertzog et al, and assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference, that a background energy spectrum be generated from gamma rays detected during periods between neutron bursts and be utilized to provide one or more standard background spectra for use in the analysis of the inelastic scattering gamma ray spectra. The standard background spectra is then updated on a repetitive basis to reflect the current background component in the detected inelastic scattering gamma ray spectrum. The measured inelastic spectrum is thereafter analyzed by comparing it with a composite spectrum, made up of standard spectra of constituents, including the background spectra, postulated to comprise the formation, to determine the proportions in the formation of the postulated constituents.
The spectral standards, except for a background standard, as employed in the aforementioned Hertzog, et al application are generated illustratively, in known laboratory formations or test pits at standard conditions of temperature, pressure and detector resolution. The measured spectrum, on the other hand, is obtained in borehole wells having temperatures which vary from borehole to borehole as well as along the length of any one borehole. As a result of such temperature variations and the age of the detector crystal, the output of gamma-ray detectors employed in obtaining the measured spectrum is subject to variation and deterioration in resolution. For example, where a sodium iodide (NaI) detector is employed, the spectral resolution of the output is known to deteriorate (peak width increase) from a measured seven percent peak full width at half maximum (0.662 Mev) at 20.degree. C. (room temperature) to over ten percent peak full width at half maximum at 150.degree. C.
Since the derivation of the foregoing information concerning constituents of the formation, depends upon accurate constituent analysis of the formation gamma ray spectra, the weight coefficients for the standard spectra which give the best fit of the composite spectrum to the unknown spectrum, e.g., as determined in accordance with the Moran, et al technique, will not, in effect, represent the relative proportions of the constituents in the formation if detector resolution is significantly different from the resolution of the standard spectra.